1. Field of the Invention
The invention relates to an adhesive resin composition, an adhesive film using the adhesive resin composition and a flat cable using the adhesive film.
2. Description of the Related Art
Flat cable is a cable composed of plural rectangular conductors arranged in parallel and two adhesive films sandwiching and covering thereof, and has characteristics of small in thickness and excellent in flexibility. Such flat cable are widely used as an internal wiring cable for office automation equipments such as printers and scanners, computer devices, video equipments such as flat-screen TVs, audio equipments, robots, various electric and electronic equipments such as ultrasonic diagnostic equipments and vehicles. Especially in case of using as an internal wiring material of electronic equipments, it is necessary to meet the UL standard and high flame retardancy is required. Also in an automotive application, flame retardancy is often required.
The adhesive film covering the plural rectangular conductors in the flat cable is often formed by wet-coating a base film with an adhesive dissolved in a solvent. Engineering plastic films excellent in heat resistance and chemical resistance are used for such a base film. Among others, polyethylene terephthalate (PET) films widely available in market and excellent in cost or supply stability are particularly preferably used.
In order to improve adhesion between a PET film and an adhesive, a surface of the polyethylene terephthalate film to which the adhesive is applied is corona-treated or UV-treated.
Meanwhile, methods of imparting flame retardancy to the flat cable include a method of rendering an insulating film itself flame-retardant and a method of rendering an adhesive flame-retardant.
One of the methods of rendering the insulating film itself flame-retardant is to use a film made of a self-extinguishing resin such as polyimide. However, films formed of a self-extinguishing resin as described above are very expensive and are only used for special purposes. Therefore, in many cases, flame retardancy is imparted by adding a flame retardant to an adhesive.
As a base of a resin for forming an adhesive layer (an adhesive layer-forming resin), thermoplastic polyester-based resins having good adhesion especially to polyethylene terephthalate used as a base material are widely used. Such thermoplastic polyester-based resins are categorized into amorphous and crystalline resins. Amorphous thermoplastic polyester-based resins are well dissolved in general organic solvents such as toluene or methyl ethyl ketone and are thus widely used as an adhesive layer-forming resin of general-purpose flat cable.
Thermoplastic polyester-based resins having high adhesion, especially amorphous thermoplastic polyester-based resins, have low heat resistance due to glass transition temperature as low as room temperature or less and is sometimes used after crosslinked with an isocyanate compound since it is difficult to use as-is for heat-resistant purpose. However, there is a problem that it is difficult to control degree of cross-linking due to high reactivity of isocyanate.
Therefore, a resin having a high glass transition temperature is blended in order to enhance heat resistance in a stable manner.
Meanwhile, crystalline thermoplastic polyester-based resins have good heat resistance and are applicable as an adhesive layer-forming resin of heat-resistant flat cable. Resin compositions formed using crystalline thermoplastic polyester-based resin such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT) have been proposed in order to improve heat resistance (see, e.g., JP-A-H04-178413).